This video has all the locations of every heart piece in The Legend of Zelda: Oracle of Seasons.

Heart Piece #1 (0:00):
Obtained randomly from Maple. This is completely random and I recommend getting at the very first encounter since she seems to drop it more often than not at that point for whatever reason. Easiest way to tell if she may drop it is if she makes an 'S' pattern when the shadow is on the ground. Good luck and savestate away!

Heart Piece #2 (0:27):
Found in Horon Village. You can get to this after you get the Seed Satchel from Gnarled Root Dungeon.

Heart Piece #3 (0:43):
Found in Woods of Winter. Come back here after you get the Power Bracelet in Snake Remains.

Heart Piece #4 (1:00):
Found in Eastern Suburbs. You can get to this heart piece after you get the Rod of Seasons with the Winter blessing.

Heart Piece #5 (1:26):
Found in Subrosia Market. Have 20 ore chunks and 10 ember seeds? You can get a nice rare peach stone for some.

Heart Piece #6 (1:46):
Found in Moblin's Keep. Defeat King Moblin and you'll find a chest with the heart piece in the remains of what used to be his crappy fort.

Heart Piece #7 (3:05):
Found in Mt. Cucco. Drop down the cliff in front of Dancing Dragon Dungeon to find an easy to get heart piece.

Heart Piece #8 (3:18):
Found in Eyeglass Lake. Turn the season to Summer and go into the hidden cave hiding under a rock in the drained lake to find this heart piece.

Heart Piece #9 (4:04):
Obtained randomly from Gasha Trees. Another random heart piece to get. Best places to get this heart piece are Tarm Ruins, Eyeglass Lake, and the Goron Mountain area.

Heart Piece #10 (4:11):
Found in Spool Swamp. Change the season to Spring and head down to the heart piece that was once surrounded by rocks.

Heart Piece #11 (4:55):
Found in the graveyard. Once you beat Explorer's Crypt, change the season to Fall and use the Roc's Cape to get to the graveyard. Lift up the mushrooms to get to this heart piece.

Heart Piece #12 (5:47):
Found in Temple Remains. After the remains turns into a lava wasteland, you can find this heart piece hidden in a cave near the warp.